The present invention relates to calibrating the exposure of cameras, more particularly to a method for setting the exposure parameters for secondary camera installations after a first camera has been satisfactorily calibrated, using a light level meter.
Photographic cameras must be adjusted to regulate the light entering into the chamber housing the unexposed photographic film in order that proper exposure is achieved. It is very well known in the art that a number of parameters must be adjusted, in combination, so that the desired exposure occurs: the illuminating light level may be varied up or down; the f-stop in the camera lens may be varied to allow for a larger or smaller light entry aperture; the shutter speed may be increased or decreased; and finally, a film having a higher or lower exposure speed, or ASA rating, may be utilized.
In order to be able to set the above enumerated exposure control parameters, a light level meter is commonly used which gives a reading of the intensity of the illuminating light. With this information, and a foreknowledge of the desired photographic effect that is desired, the exposure parameters may each be set with confidence that the photographic film will be properly exposed.
Various devices have been used in the prior art to sample illuminating light. Most typically, these include a light level meter having a photo-sensitive surface which converts light energy into electrical energy. The electrical energy in turn is used by an electrical circuit in the meter to register the light level in either analog or digital form.
The problem encountered by these light level meters is that they are somewhat bulky, preventing them to be used in places having very limited room, such as a camera used for taking photographs of items in an entirely self contained space. An example of this is a photographic camera used to photograph documents onto microfilm.
There is, consequently, a need to be able to selectively sample light in hard to reach locations due to limited spatial accessibility. U.S. Pat. No. 4,194,838 to Bey, et al, discloses the use of a light probe coupled to a light guide, which in turn is connected to a photodetector circuit; this patent teaches this structure for use in a digital color densitometer used to set the exposure of photographic paper, where the scene illuminated is an exposed photographic negative. U.S. Pat. No. 4,061,428 to Amano, et al, discloses a scanning color densitometer for obtaining information on the areal distribution of optical density of a photographic image on a negative using plural input fiber optic bundles. Both of these prior art solutions are directed to setting the exposure of densitometers where the illuminated object is a photographic negative, and neither is directed to solving the need to sample light intensity in hard to reach places. A means for setting the exposure for a standard camera taking a photograph of a scene where the light sampling must be accomplished in a hard to access location remains a problem in the prior art.
What has been discussed thus far relates to the set-up of an individual camera. When a plurality of identical cameras are to be calibrated, the procedure for adjusting each independently can be extremely cumbersome, time-consuming and expensive. Typically, each camera is independently calibrated by taking a light reading, then the camera is adjusted in terms of its f-stop and shutter speed for the particular film being used. In repetitive operations, such as in microfilm camera installations where the f-stop, shutter speed and film type are predetermined, a common calibration procedure is to take a series of film exposures over a range of illumination light level settings to ensure proper exposure of the film. Difficulty enters when the light intensity later varies either because the lamp in the the source of illumination is subject to a changed line voltage, the lamp has been replaced, there is variation in surface texture, coloration, etc. of objects in the scene, or the optics in the light optical path of the camera apparatus have been adjusted or replaced; yet another source of difficulty arises when the film is changed, either in terms of the brand used or the ASA rating. To ensure that the film is properly exposed, a series of test exposures must again be taken. Test exposures result in equipment down-time while the technician awaits developing at a film laboratory, but is necessary so that he can finely adjust the camera as indicated by the best exposed photograph in the series.
What is needed is a method that allows the technician, to set-up each camera, whenever the need arises, such as a new installation, or a change in illumination, film type or camera optics, without the need for test exposures to ensure the accuracy of each camera calibration, based upon the results known for a reference camera.